Diagnostic Ultrasound




Sound waves having frequency more than 20,000 Hz are called ultrasounds because they are not audible to the human ear. Frequencies between 1 to 10 MHz are mainly used for the purpose of diagnostic ultrasound. This diagnostic technique uses the pulse-echo principle for imaging.

            The piezoelectric crystals present in the transducer/probe of the ultrasound machine have the duel properties i.e. (a) first, they produce sound waves which travel through the body and then either absorbed, scattered or reflected back when they strike with any surface/interface (b) they change the reflected sound waves into electric impulses which then are displayed on the screen after processing.

FATE OF ULTRASOUNDS


  1. Absorption: when the energy of sound waves has absorbed the tissue, they can’t return to the probe. This forms the principle for therapeutic ultrasound.
  2. Reflection: sound waves after striking an interface return back towards the transducer and collected there by the piezoelectric crystals. This forms the principle for diagnostic ultrasound.
  3. Scattering: when a beam encounters an interface that is irregular and smaller than the sound beam they are scattered in all directions.

Once the beams are reflected and caught by the transducer these are converted into electrical signals. These signals are measured and displayed as the measure of the amplitude of echo. This is known as echo quantification. Different modes are used to visualize this echo quantification in diagnostic ultrasound i.e. A, B, and M modes.


TRANSDUCERS


            The frequency of a transducer is calculated by the times a piezoelectric crystal expands and contracts in one second. A higher frequency penetrates less far but provides better resolution while a low frequency penetrates far. In veterinary practice transducers of 3.5, 5, and 7.5 MHz are most commonly used. 7.5 MHz transducer is used for scanning of very superficial organs while a 5 MHz transducer is used for the interfaces lying not so deep. 3.5 MHz transducer is used for the scanning of deep-lying structures.

Following are the main types of transducers used in veterinary practice:



1.      Linear Array: these transducers are prepared in a rectangular shape having piezoelectric crystals positioned side by side. Each crystal produces sound waves. The beam thus produced is rectangular in shape and thus permits a good visualization of superficial surfaces. This type of transducer is mostly used in veterinary reproduction per rectum scanning of reproductive organs.

2.      Convex Arrays: in this type of transducer the crystals are produced in a curvilinear fashion. The beams produced by this type after emitting from a crystal does not travel in a straight line but expand along its axis. Thus by placing the transducer on one part we can have the image of the greater area beneath.

3.      Sector Arrays: these transducers contain a single crystal, which oscillates or rotates to produce a fan-shaped beam. This type is used for scanning organs present in the thoracic cavity e.g. heart though a small contact area.


IMAGE DISPLAY


1. A-Mode (Amplitude Mode)

            In this display system the two parameters of echoes are displayed i.e. distance from the probe and the amplitude. The horizontal line shows the distance and the amplitude is displayed in form of vertical lines.

2. B-Mode (Brightness Mode)

            In this mode, the amplitude of each wave is displayed as a white dot on the screen. A two-dimensional image of the area covered by the beam is produced in this display system. The picture presents a slice through the body in the plane of the beam.

3. M-Mode (Motion Mode)

            This mode is used for scanning the moving organs inside the body. It records the position and motion of the echo. The images thus obtained are moved along a horizontal axis showing the movement of the structure along that line. A stationary interface e.g. bones will produce a straight line and a moving interfaces e.g. heart will produce wiggly lines.


SCANNING PROCEDURE


¨                Hairs should be clipped properly before scanning of an area, because they entrap air          between them, which doesn’t allow ultrasounds to travel.

¨                Application of gel or oil to minimize the air between the transducer and contact surface

¨                Restrain and anesthetize the animal if required to perform the scanning with a free mind

¨                Keep in mind proper anatomy of the animal under consideration otherwise you will come out with nothing.


Image interpretation


            Images are usually displayed as white against a black background. Various terms used to describe the image are as follows:



  1. Hyperechoic or echogenic:

These present the bright echoes, which appear as a white on conventional scans. Such images are given by high reflective interfaces such as bones.



  1. Hypoechoic:

These appear as grey images or dark screens and are by interfaces of moderate reflection such as soft tissue.



  1. Anechoic or echolucent:

In the absence of an echo, the image is seen as black. It is represented by the complete transmission of sound such as though fluids. The image formed on the scan screen is actually a mixture of the images of different echoes depending upon the area scanned. Fluids will give an anechoic image as the sound beam passes uninterrupted.



There is often a normal bright area immediately deep to fluid and this phenomenon is called acoustic enhancement. Similarly, bone or gas or mineral deposits reflect the sounds waves totally and the image seen is bright with no visible structure beneath it. This phenomenon is called acoustic shadowing and helps in the detection of urinary and biliary stones. Soft tissues present an image of mixed shades of grey depending upon their proportion of fat, fibrous tissue, and fluid.


ARTIFACTS


            A sonologist should be aware of the common artifacts to avoid errors in image interpretation. These artifacts are:



  1. Acoustic Shadows:

These are caused by attenuation or reflection of the sound beam at an acoustic interface. When this happens, the pulse is unable to reach the deep interface to produce any echo. In order to cast a shadow, the interface must reflect a large percentage of the sound beam. The most common acoustic shadows of clinical significance are those caused by cystic, renal, and biliary calculi. The gas causes near-total reflection of the beam.



  1. Reverberation:

      The largest sources of positive artifact echoes that are not real. When the sound beam arrives back at the transducer, a portion of the sound beam is absorbed by the transducer crystal to produce a small electrical pulse that records the echo. The remainder, however, is reflected back into the patient. So the echo bounces back to the transducer and is again reflected through the patient and back to the transducer. This process of echo bouncing back and forth between the two interfaces is known as reverberation.



  1. Mirror Image:

This effect occurs at highly reflected interfaces. Returning echoes reach the transducer under a time delay and are registered on the image as being a highly echogenic interface that is in the path of the beam.



  1. Comet Tail:

This is caused by a highly reflective interface most commonly the air-fluid interface. Comet tail occurs most commonly in partially consolidated lung at the interface between the diaphragm and lung, and at the interface between the bowel wall and bowel gas.

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